In a TN (twisted nematic) type liquid crystal display device, variations in the gap of a liquid crystal cell have an immense influence on display qualities such as a response speed, a viewing angle and a contrast, and cause a display nonuniformity. In an STN (super twisted nematic) type liquid crystal display device, it is considered that the variations in the gap of a liquid crystal cell is required not to exceed 0.05 .mu.m, and the accuracy of a spacer dominates the display qualities to a great extent.
The variations in the gap of a liquid crystal cell can be decreased by increasing the dispersion density of a spacer dispersed in the liquid crystal cell. By this means, however, the leakage of light from a spacer increases if the spacer has light transmittance properties. It is therefore general practice to color the spacer itself in black, or the like, for preventing the light transmittance through the spacer.
The above technique includes a method in which an organic substance is introduced into metal oxide fine particles obtained by hydrolysis of a metal alkoxide such as silica fine particles, in the presence of an alkali metal and the resultant product is treated at a temperature of at least 250.degree. C. to blacken it as proposed in JP-A-63-89890 or a method in which silica fine particles are brought into contact with a fluorinating agent and an organic solvent to introduce an organic substance into the silica fine particles and the resultant product is heated to 500.degree. C. or higher to blacken it as proposed in JP-A-3-279209.
In the method disclosed in JP-A-63-89890, however, it is difficult to control the residual amount of the organic substance and it is therefore difficult to adjust the blackness. Further, since the alkali metal used for introducing the organic substance into the silica fine particles is together taken into the silica fine particles, it is required to extract and remove the alkali metal taken into the silica fine particles by bringing the silica fine particles into contact with a mineral acid after the introduction of the organic substance. Further, the method disclosed in JP-A-3-279209 has a problem that hydrofluoric acid is generated at a heating time so that the material of a calcining furnace is corroded to cause the inclusion of impurities.
In view of these points, the present inventors sought to develop a method in which an alkali metal is not taken into a spacer and no corrosive gas occurs at a calcining time. As a result, it was found that black fine particles can be obtained by adding a titanium compound (titanium alkoxide, etc.) which is formable into an titanium oxide coating on the surface of silica fine particles to a dispersion of silica fine particles in a water-alcohol solvent, hydrolyzing the titanium compound to form a titanium oxide coating on each silica fine particle, separating the titanium oxide-coated silica fine particles from the solvent, then freeze-drying the titanium oxide-coated silica fine particles and calcining them in a reducing atmosphere such as ammonia gas to blacken the above titanium oxide coatings, and it was proposed to use the so-obtained black fine particles as a spacer for a liquid crystal display device (Japanese Patent Application No. 3-184002 (JP-A-5-9027)).
In the method disclosed in the above JP-A-5-9027, when the time for reduction-treating the titanium oxide coatings in an ammonia gas atmosphere is increased or when the reduction-treating temperature is increased to obtain black fine particles having a higher blackness, the amount of a formed titanium nitride increases. As a result, the resultant black fine particles have decreased electrical insulation properties and comes to show electrical conductivity. The present inventors therefore sought to develop black fine particles having higher electrical insulation properties. As a result, it was found that black fine particles having a high blackness and excellent electrical insulation properties (black fine particles each of which is provided with an electrically insulating coating will be referred to as "insulator-coated black particles" hereinafter) can be obtained by hydrolyzing and polycondensing a metal alkoxide, etc., in a water-alcohol solvent in the presence of ammonia to form an electrically insulating coating of a metal oxide (silica, etc.) on the surface of each of black fine particles produced according to the method disclosed in the above JP-A-5-9027. The use of the above insulator-coated black particles as a spacer for a liquid crystal display device was also proposed (JP-A-5-257150).
Further, although not directly intending to obtain a spacer for a liquid crystal display device, the present inventors developed a method which enables the production of electrically conductive fine particles, comprising calcining a titanium oxide coating formed on the surface of each of metal oxide particles in a reducing and/or nitriding atmosphere to converting the titanium oxide coating to an electrically conductive coating containing partially reduced titanium and/or titanium nitride, wherein the titanium oxide coating formed on the surface of each particle by one reaction has a greater thickness than the coating formed by the method disclosed in the above JP-A-5-9027 and the adhesion between the above coating and each particle is higher than the adhesion between each particle and the titanium oxide coating turned black in the black fine particles obtained by the above JP-A-5-9027. The use of the electrically conductive fine particles obtained by the above method in an anisotropic electrically conductive film or the like was proposed (JP-A-6-162817).
In the method disclosed in the above JP-A-6-162817, electrically conductive fine particles as an end product are obtained as follows. Particles of a metal oxide are dispersed in an alcohol-based solvent mainly containing an intermediate alcohol having 4 to 10 carbon atoms, an alkaline aqueous solution is added to the dispersion to activate the particle surfaces, a titanium compound which can form a titanium oxide is added to the mixture, the titanium compound is hydrolyzed to form a titanium oxide coating on each of the above particles, then, the solid and the liquid are separated from each other, the solid is freeze-dried to obtain titanium oxide-coated particles, and the titanium oxide-coated particles are calcined in a reducing and/or nitriding atmosphere to form coatings containing partially reduced titanium oxide and/or titanium nitride.
Meanwhile, a spacer for a liquid crystal display device is required to have excellent electrical insulation properties for preventing useless conduction among a great number of electrodes provided in a liquid crystal cell. It is therefore improper to use the electrically conductive fine particles obtained by the method disclosed in the above JP-A-6-162817 as a spacer for a liquid crystal display device. Further, the insulator-coated black particles disclosed in the above JP-A-5-257150 are suitable as a spacer for a liquid crystal display device in respect of electrical insulation properties since they have electrically insulating coatings. Since, however, the titanium oxide coating formed on each particle by one reaction in the production process thereof has a small thickness, the productivity is low when it is attempted to finally produce insulator-coated black particles having a high blackness by forming titanium oxide coatings having a large thickness. The same is also true of the method disclosed in JP-A-5-9027. A titanium oxide coating having a large thickness can be formed by the method disclosed in the above JP-A-6-162817. As already described, however, it is the method of producing electrically conductive fine particles that is disclosed in the above publication.
Further, with regard to a titanium oxide coating formed on each of particles, the titanium oxide coating is increasingly desired to be denser and to have a larger thickness. With regard to a case where electrical insulation properties are imparted on the basis of an electrically insulating coating, it is also increasingly desired to form a denser electrically insulating coating having a larger thickness for obtaining higher electrical insulation properties.
It is a first object of the present invention to provide a process for the production of insulator-coated black particles in which a titanium oxide coating formed on the surface of each particle by one reaction has a large thickness and finally obtained particles have a high blackness.
It is a second object of the present invention to provide a process for the production of black particles each of which has a blackened titanium oxide coating which is denser and has a large thickness.